1. Field of the Invention
The present invention relates to electronic circuits and, more specifically, to an electronic circuit that may be selectively disabled.
2. Description of the Prior Art
Many modern semiconductor chips have long service lifetimes and low rates of failure. In most applications, this is a desirable quality. However, in certain applications it is more desirable for a chip to have a limited or controllable service life. For example, many electronics companies provide evaluation copies of their circuits to potential customers to allow the customers to evaluate the circuits. Once the evaluation period has ended, the company providing the circuit usually has to rely on the good faith of the potential customer to return or destroy the circuit. In another example, chips used in military hardware that is transferred to certain foreign forces whose long term loyalty cannot be assured should be capable of being disabled once the time for their intended purposes had passed.
There is also a concern that users of certain circuits might to tamper with the circuits to use them for unauthorized purposes. The most common way of trying to prevent such tampering is to place a sticker on the housing for the circuit with a warning that tampering will void the warranty. This approach may be ineffective in many cases.
Therefore, there is a need for a circuit that can be disabled when the user is attempting to use the circuit outside of normal operating conditions.